1. Field of the Invention
The present invention is concerned with displays, particularly passive matrix displays, driven by pulsed driving conditions. The present invention is further concerned with new semiconductive polymers that may be used in such displays. The present invention is still further concerned with the lifetime properties of semiconductive polymers, and in particular increasing the lifetime of a semiconductive polymer in a display driven by pulsed driving conditions.
2. Related Technology
Many displays consist of a matrix of pixels, formed at the intersection of rows and columns deposited on a substrate. Each pixel is a light-emitting diode (LED), such as a polymer LED (PLED). With reference to FIG. 1, the architecture of an LED comprises a transparent glass or plastic substrate 1, an anode 2 and a cathode 4. An electroluminescent layer 3 is provided between anode 2 and cathode 4.
Coloured displays are formed by positioning matrices of red, green and blue pixels very close together. To control the pixels, and so form the image required, either ‘passive’ or ‘active’ matrix driver methods are used.
Active matrix displays incorporate a transistor (TFT) in series with each pixel which provides control over the current and hence the brightness of individual pixels. Lower currents can flow down the control wires since these have only to programme the TFT driver, and the wires can be finer as a result. Also, the transistor is able to hold the current setting, keeping the pixel at the required brightness, until it receives another control signal. DC driving conditions typically are used for an active matrix display.
In passive matrix systems, each row and each column of the display has its own driver, and to create an image, the matrix is rapidly scanned to enable every pixel to be switched on or off as required. The controlling current has to be present whenever the pixel is required to light up.
As described in Proc. of SPIE Vol 2800 (2003) “Organic Light-Emitting Materials and Devices”, the principle of passive matrix addressing is rather straightforward and uses the eye's insensitivity to the fast re-positioning of light generation. Instead of simultaneously addressing all the pixels needed to display an image frame, in the passive matrix approach, the different pixels are addressed after each other via a row to row scrolling. The intensity of the short illumination is much more intense than when all the pixels are generating light for the total frame time (the number of rows times the average overall brightness needed). If the refresh rate of the whole frame is high enough, the human eye observes the scrolled picture as a standing picture with an average brightness. The advantage of passive matrix driving is the simple substrate structure allowing for easy customization and low substrate costs. Pulsed driving conditions typically are used for a passive matrix display.
Synthetic Metals 91 (1997) 3-7 and Synthetic Metals 113 (2000) 155-159 provide information on the structure of passive matrix organic LEDs, the contents of which are hereby incorporated by reference. Reference is made in particular to FIG. 11 (c) in Synthetic Metals 91 (1997) 3-7, which shows deposition of the organic emitter layer by evaporation, followed by evaporation of the cathode. Solution deposition (e.g. inkjet printing) of the emitter layer is equally applicable to this structure. The background section of the Synthetic Metals 113 (2000) 155-159 article provides detail on how the cathode is patterned into stripes using a photoresist material.
An important parameter is the lifetime of a display. Inadequate lifetimes are a particular problem for blue light-emitting polymers.
WO 02/092723 and WO 04/083277 are both concerned with blue light-emitting polymers for use in optical devices. Both disclosures refer to blue emissive repeat units comprising an optionally substituted repeat unit of formula:
                wherein each R′ is independently selected from hydrogen or a solubilising group. Particularly preferred solubilising groups are optionally substituted alkyl or alkoxy. Most preferably, R′ is n-butyl.        
Both disclosures also refer to an optionally substituted repeat unit of formula:
                wherein preferably, each Ar is independently selected from the group comprising an optionally substituted residue of formula:        
                wherein n=1, 2 or 3 and R is a solubilising group or hydrogen. Particularly preferred groups R are hydrogen and optionally substituted alkyl or alkoxy. Most preferably, R is hydrogen or butyl. It is said that by “butyl” is meant n-, sec- or tert-butyl.        
In the Examples in WO 02/092723 and WO 04/083277, only polymers P1 to P4 containing 9,9-diphenylfluorene repeat units were made and compared with a comparative polymer where the 9,9-diphenylfluorene repeat units were replaced with 9,9-di-n-octylfluorene repeat units.
WO 02/092723 primarily is concerned with increasing the thermal stability (Tg) of polymers. WO 04/083277 is concerned with improving device lifetime but teaches to omit TFB from the polymer to achieve this. Neither WO 02/092723 nor WO 04/083277 is concerned particularly with pulsed driven devices and neither disclosure even mentions pulsed driven devices.
EP 1394188 is concerned with improving the lifetime of polymer compounds comprising a repeat unit:

However, EP 1394188 teaches to use a polymer compound comprising a repeating unit shown by formula (1) or (2) on page 4 of EP 1394188 in order to improve the lifetime. In these formulae, the terminal aryl groups (E1, E3, E8 and E9) each has three or more substituents.
According to Proc. Of SPIE Vol 2800 (2003) “Organic Light-Emitting Materials and Devices” lifetime measurements are mostly performed at dc driving. However, it is said that the lifetime of light-emitting materials for passive matrix applications should also be tested using the pulsed driving conditions experienced in full colour displays.
Despite recent advances in the lifetimes of polymers for use in organic light-emitting diodes, there remains a need for alternative polymers for use in pulsed driven and dc driven displays comprising an organic light-emitting diode, preferably polymers with an improved lifetime. In this regard, it is the particular problem of the present invention to improve the lifetime in pulsed driven displays of semiconductive polymers comprising one of the repeat units:
